The magnetometer of the invention falls within the category of resonance magnetometers described in the article by F. HARTMAN and entitled "Resonance Magnetometers" published in the journal "IEEE Transactions on Magnetics", vol. MAG-8, No 1, March 1972, pp. 66-75.
A resonance magnetometer is a device which, when plunged into a magnetic field Bo, delivers an electric signal with a frequency F and whose value is linked to Bo by the LARMOR equation: EQU F=.gamma. Bo
where .gamma. is a gyromagnetic ratio (of an electron or nucleon depending on the substance used). For an electron, this ratio is equal to 28Hz/nT, for example.
In this category of devices, the magnetometer with optical pumping occupies an overriding place. The general constitution of this magnetometer is diagrammatically shown on FIG. 1.
A cell 10, at least partly transparent, is filled with a gas 12, generally helium (isotope 4) at a pressure of between 1 and several torrs. A luminous source 14 delivers a luminous beam 15 whose wavelength is situated around 1.1 .mu.m if helium is used. This beam is injected into the cell 10.
Furthermore, a weak or mild radiofrequency discharge is produced in the gas by suitable means (not shown). This discharge produces atoms in a metastable state (2.sup.3 S.sub.1 in the case of helium). The incident luminous beam 15 "pumps" these atoms from the metastable state so as to bring them into another excited state (2.sup.3 P).
In the presence of a magnetic field Bo, the energy levels are separated into sub-levels known as ZEEMAN sub-levels. A radiofrequency resonance between these sub-levels may be established by a radiofrequency field (magnetic resonance) or by modulation of the light (optical double resonance). In the case of helium and for the isotope 4, resonance is established between two ZEEMAN electronic sub-levels of the metastable state. This resonance is revealed by various known types of electronic means and shown on the diagrams on FIG. 1 by a photodetector 16, processing means 18 and a device 20 to measure the frequency of the processed electric signal. The present invention does not concern these means and shall not be described in detail as any suitable device shall suffice. For example, reference may be made to the set of four patents lodged by the current Applicant, namely FR-A-2 663 429, FR-A-2 663 430, FR-A-2 663 431 and FR-A-2 663 434 which describe various dispositions able to be used to measure the frequency F.
Once the frequency F has been measured, the value Bo of the ambient magnetic field is instantly deduced by the ratio Bo=F/.gamma..
The luminous source used in this type of application needs to have special characteristics.
Firstly, it needs to emit at a wavelength tuneable around the 2.sup.3 S.sub.1 -2.sup.3 P of the helium. This wavelength then needs to be stabilized on one of the lines of this transition, for example the line Do in the case of the isotope 4 of the helium.
Furthermore, this implies that the laser, used to constitute this source only emitting this single wavelength as wavelength modes differing from that of the selected transition, would result in degrading the performances of the magnetometer: increasing the amplitude noise of the laser, appearance of modal noise, possible resonance frequency shifts of the magnetometer linked to movements of the energy levels of the helium atoms under the effect of the non-optical interaction, reduction of the efficiency of optical pumping, etc.
The entire energy emitted thus needs to be within the wavelength range corresponding to the selected transition which, for example, corresponds to a frequency band of about 2 GHz at ambient temperature for the line Do of the helium.
Finally, the source needs to be insensitive to its thermic and mechanical environment.
For all these reasons, traditional helium lamps have been replaced by optically pumped solid lasers. Thus, the document FR-A-2 598 518 (or its European counterpart EP-A-O246 146) concerns a magnetometer where the laser is an LNA laser (lanthane-neodyme aluminate laser). Reference may also be made to the article by L. D. SCHEARER et al and entitled "Tunable Lasers at 1080 nm for Helium Optical Pumping" published in J. Appl. Phys. 68 (3) on 1 Aug. 1990, pp. 943-949 or even the article by L. D. SCHEARER et al entitled "LNA: A New Cw Nd Laser Tunable Around 1.05 and 1.08 .mu.m" published in IEEE J. of Quant. Electronics, vol. QE-22, No 5, May 1986, pp. 713-717.
In these prior techniques, the solid laser used is associated with a large number of devices, as shown diagrammatically on FIG. 1. In the example represented, the solid laser, which is optically pumped by a source LD associated with a focusing and formation optic system L1, L2, includes:
a crystal A constituting the amplifier environment; this crystal may consist of LNA and be about 5 mm long with a diameter of 5 mm: multidielectric layers (reflection coefficient exceeding 99% at 1.08 .mu.m and transmission coefficient of more than 95% for the pumping light) are deposited on the front face of this crystal so that the latter also constitutes an inlet mirror M1 of the laser cavity;
an outlet mirror M2; a dichroic treatment of this mirror is able to force the emission of the laser onto the band centered around 1.03 .mu.m; the LNA fluorescence spectrum in fact has two bands around 1 .mu.m, the most intense being situated close to 1.054 .mu.m; the retained multidielectric treatment (transmission of 50% for .lambda.=1.05 .mu.m and transmission of about 1.5% for .lambda.=1.08 .mu.m) makes it possible to prevent the laser from oscillating spontaneously close to the wavelength of the main fluorescence band; it is also possible to use a LYOT filter given the reference LY on FIG. 1;
one or several solid FABRY-PEROT type standards with a thickness of several hundreds of microns (150 .mu.m and 200 .mu.m, for example) inserted in the cavity; these FABRY-PEROT standards play the role of selective wavelength elements; control of their thickness and/or their inclination (that is, control of the optical length to which they correspond) makes it possible to wavelength-tune the laser;
finally, so as to finely tune this emitted wavelength, a piezoelectric shim P2 equips the mount of the outlet mirror M2 of the cavity; it is then possible to slightly make the length of this cavity vary and have the wavelength of the laser coincide with the center of the absorbtion line retained (for example, the line Do) ; it is generally by this means that wavelength control of the laser is effected from a flourescence measurement of a helium cell.
In other embodiments, described in the article by L. D. SCHEARER et al already referred to, the crystal constituting the amplifier environment may be replaced by an optical fiber doped with neodyme. The general structure of the device remains identical to that described above for the LNA laser.
The major drawback of these magnetometers lies in the difficulties encountered in stabilizing the emission wavelength of the laser. This wavelength is in fact determined by a set of elements, namely:
the total length of the cavity (which fixes the place of the modes of the cavity),
the length of the amplifier environment (crystal or doped fiber) which often constitutes a FABRY-PEROT resonator giving rise to a parasite standard effect,
the thickness and inclination of the selective elements (E1, LY).
So as to obtain the laser effect concerning the desired transition, it is advisable that all these parameters be simultaneously adjusted, the variation of any one of them being immediately expressed by a variation of the wavelength emitted by the laser and a deterioration of the performances of the magnetometer.
Owing to this, the magnetometers using such lasers are extremely sensitive to the environment, especially to mechanical vibrations or thermic variations. In fact, a temperature variation is expressed by a dilatation or shrinkage of the materials, which accordingly shifts the wavelength. Thus, for example, for the LNA crystal, a temperature variation .DELTA.T is expressed by an index variation of about 2.1-10.sup.-5 .DELTA.T.
The same applies for the FABRY-PEROT blade, these two parameters further modifying the total length of the cavity. In practice, this is finally expressed by the need to control the temperature of the various elements of the cavity with a precision of about several hundredths of a degree.
Similarly, it is essential to automatically control the total length of the cavity by moving the outlet mirror.
Finally, it may be observed that these lasers are expensive, that is exceeding 100 kF (about $30,000.00) in the current state of the art.